A coating system becoming increasingly popular, particularly in the automotive industry, is one known as "color plus clear." In this system the substrate is coated with one or more applications of a pigmented basecoating composition to form a basecoat which thereafter is coated with one or more applications of an essentially clear topcoating composition to form a topcoat.
However, there are several disadvantages with known color plus clear coating systems. After conventional basecoating compositions are applied to the substrate a rather long period of time, on the order of about 30 minutes or more, may be required between the application of the conventional basecoating composition and the conventional topcoating composition. Such a period is needed to prevent adverse attack by components of the conventional topcoating composition, particularly solvents, on the basecoating composition at the interface of the two, a phenomenon often referred to as strike-in. Strike-in adversely affects the final appearance properties of the coated product. Strike-in is an especially serious problem when metallic-flake pigments are employed in the basecoating composition. Strike-in, among other things, can destroy the desired metallic-flake orientation in the basecoat.
Often, known color plus clear systems based on thermosetting resins require elevated temperatures typically of at least 120.degree. C. for curing. It would be desirable to provide a color plus clear coating method in which relatively low temperatures, for example, below about 82.degree. C., and preferably ambient temperatures, could be utilized. Previous attempts to develop such coating systems resulted in systems which had the disadvantages of being too time consuming and/or energy intensive or resulted in cured films which were deficient in various combinations of physical properties.
In addition to the need for a color plus clear coating system which can utilize low temperature curing, it would be desirable that the system not require the use of organic isocyanates. A number of known color plus clear coating systems involve one or more isocyanates in one or more steps of the coating procedure. Recent studies have suggested that overexposure to organic isocyanates may pose health problems.
In accordance with the present invention, a color plus clear coating system has been developed which can provide an acceptable rate of cure at low or even ambient temperatures and results in coated products in which the films exhibit an excellent combination of good appearance and physical properties such as good solvent resistance, high gloss, excellent gloss retention, good durability, good visual appearance of depth, substantial absence of strike-in, and good metallic pattern control when metallic-flake pigments are employed. Additionally the color plus clear system of the present invention can be utilized with either a reduction of or even elimination of the use of organic isocyanates without sacrificing the attendant advantages of the present invention.